FWIW, when I started getting serious in detecting (2010), I would find a silver coin about one every half hour or so. Now that rate, for me, is about one every 2 hours. My detector is the same, and my skill has improved, so it is diminishing, at least here in Chester County.
It seems obvious, at least to me, that that rate will continue to get worse, to the point were it is no longer fun for me, since I do this for one thing, and one thing only, and that is to find silver coins. When that will occur, I don't know. One per 2 hours is still fun for me.
They do regenerate (due to roots, ice, animals pushing previously missed coins into different positions, etc.), but the amount of coins opened up because of this is pretty low, IMHO (I do plan to go over my 50 to 100+ silver sites from 2011/2012 this year or next to try to figure this out, tho).
The best bet, it seems, is to wait for previously undetectable sites to come online, but even this is dicey. We had a private golf course around here that dates back to the thirties that the school district was taking via eminent domain, and I waited patiently for the litigation to end and the transfer to take (and besides, people were still playing golf on it), and when it finally became public property and I got on it, it turns out it was pretty picked over, so clearly it was being detected at night as private property, as it is unlikely permission would have been granted, and it was undetectable in the day, so you never know.
I'm not convinced the new machines can out silver an E-Trac in the hands of a skilled operator, but we'll see. The combination of FBS, auto ground balance, tone id, and target sizing is a killer combination; if the new machines have this stuff (I'm sure they have a faster processor, which of course is a positive), then maybe. I can't wait to play with one in one of my "hunted out" sites.
Speaking of "hunted out", I think people give up on those sites too quickly. There are parks around here that everyone says are "hunted out", and I go in and find 17, 20+ silvers in some of them. I'll take 'em. I do better there than permission sites, cuz at the end of the day, the parks had many more people. But yes, these places will get hunted out.
The one question is mineralization. The E-Trac does not do particularity well in heavy ground. I don't suspect any VLF machine does, but maybe the new ones do. I hear stories about it; are these new machines magic, or are they fanboy fish stories? Can't wait to try one on some of my bad dirt around here. There is a possibility for opening up some terrain.
The answer may be a completely different technology, such as ultra sound. You'll be able to read the date before you dig
Don't know if such technology will ever be viable in consumer machines, but it is possible.
Oh well, some random, incoherent rambling, but I think the OP is basically right. Just hope it doesn't happen in my lifetime. Does make me want to get off my duff and compare, side by side, the E-Trac and one of the newer machines sooner rather than later.